Outbreaks of Pfiesteria piscicida and other harmful algal blooms (HABs) are a major concern to human health and welfare in the Chesapeake Bay area and other estuarine habitats along the Atlantic coast of the United States. Often referred to as "ambush-predators", P. piscicida appears to use at least two chemically distinct toxins to attack fin-fish prey such as the Atlantic menhaden. The toxins are produced in response to chemical signal(s) released by foraging fish. These toxins cause the fish to become lethargic and injure the fish skin, resulting in open bleeding sores and hemorrhaging. The same toxin molecules also apparently affect humans who come in contact with the water or breathe aerosols from contaminated water. The effects include symptoms such as narcosis, development of sores, acute short-term memory loss and cognitive impairment. Fundamental questions regarding the mechanisms of P. piscicida toxin production remain unanswered. In our experimental approach, we will employ the tools of molecular biology to address basic scientific questions regarding aspects of Pfiesteria toxigenesis. As specific aims, we propose to: (1) Measure the effects of the epiphytic and endosymbiotic bacteria on toxin production, amplification, breakdown, or modification; (2) Develop the means to identify toxin production by the construction of whole cell bacterial luciferase biosensors; and (3). Identify the fish-derived molecular signals eliciting toxin production and/or release. The results of this research will provide much needed answers to the mechanism regarding Pfiesteria toxin synthesis, modification, and breakdown. This basic knowledge can ultimately be applied towards novel means to control and predict outbreaks of P. piscicida as well as providing novel bioassays for assessing human health effects.